LED lighting technology is developing rapidly. Especially, LEDs are becoming available at decreasing prices. For use in LED lighting appliances, there is a general desire to provide low-cost LED drivers. Reducing the cost can for instance be done by reducing the number of components, and single-stage driver architectures are preferred. However, with increasing LED power, the drivers must meet more stringent requirements relating to distortion of the line current. Although low line current distortion is feasible with single stage architectures, there often is a trade-off between load regulation and line regulation, line-current-distortion and output ripple (flicker) and the corresponding buffer size and cost.
Although it is easy to meet high power factor requirement, low total harmonic distortion and good line/load regulation, it is very difficult to solve the output current ripple (e.g. at 100 Hz or 200 Hz) even using a very large output buffer capacitor.
According to technical investigations, the issue of flicker can be considered to be solved if the output current low frequency ripple is lower than 4%. Generally a dual-stage topology that means an AC/DC Boost converter and a DC/DC flyback converter are used. This topology has a high cost and component count due to the duplication of high voltage switches and control circuits.
A well-known single-stage driver topology (or sometimes it is referred to as a 1.5 stage converter) is the BiFRED topology (Boost Integrated Flyback Rectifier/Energy storage DC/DC converter).
FIG. 1A is a block diagram schematically showing a non-isolated BiFRED converter 1 powered from the mains 2 for driving an LED load L. Reference numeral 3 indicates a rectifier, reference numeral 4 indicates an EMI filter. The actual converter comprises a series arrangement of a first diode D1, a first inductor L1, a storage capacitor C1 and a second inductor L2 connected between first and second input terminals 5 and 6. The input terminals 5 and 6 are connected to the output of the filter 4.
It is noted that the order of first diode D1 and first inductor L1 may be different. It is further noted that the order of storage capacitor C1 and second inductor L2 may be different. It is further noted that the direction of the first diode D1 determines the direction of current flow, and hence determines the mutual polarity of the input terminals. For sake the of convenience, the first input terminal 5 will be termed a “high” input terminal while the second input terminal 6 will be termed the “low” input terminal.
Reference numeral A indicates a node between first inductor L1 and the series arrangement of storage capacitor C1 and second inductor L2. A controllable switch S1 is connected between the node A and the low input terminal 6.
The converter 1 further comprises, connected in parallel to the second inductor L2, a series arrangement of a second diode D2 and a parallel arrangement of an output capacitor C2 and the LED load L. Reference numerals 9a and 9b indicate output terminals for connecting the load. It is noted that the converter can also be used for other types of load.
Reference numeral 8 indicates a control device for the switch S1. The control device controls the switch S1 to be either conductive (first state) or non-conductive (second state), and alternates between these two states at a certain repetition frequency.
The basic operation is as follows. During the first state, the switch is conductive and the first inductor L1 is charged from rectified mains via the switch S1. The energy in the first inductor L1 is magnetic energy which is proportional to the inductor current. The inductor current is increasing.
During the second stage, the switch is non-conductive, the inductor current continues to flow, discharging the first inductor L1 and charging the storage capacitor C1. The current in the first inductor L1 decreases, while the voltage over the storage capacitor C1 increases. The charging current from L1 to C1 also flows partly through the second inductor L2 and partly via the second diode D2 to power the LED and to charge the output capacitor C2.
During the first stage, the storage capacitor C1 also discharges over the second inductor L2, via the switch S1. During the second stage, the energy stored in the second inductor L2 will be used to charge output capacitor C2 and to power the LED.
FIG. 1B is a schematic diagram showing an alternative embodiment of the converter, indicated by reference numeral 11. In this alternative embodiment, the second inductor L2 is the primary winding of a transformer T1 which has a secondary winding L3 connected to the second diode D2. This is an isolated BiFRED converter. An advantage of using such transformer is that the primary and secondary windings may be mutually isolated such as to provide an insulation between the input and output, and the respective numbers of turns may have a ratio higher than 1 such as to provide a voltage increase at the output, but otherwise the operation is the same as described above.
For a correct operation of the converter, the timing of the switching moments from a first state to a second state and from the second state to the first state is important. The control device may operate at an arbitrary high frequency, but in view of the fact that the charging current is derived from rectified mains, the current in the load may have a frequency component (ripple) equal to twice the mains frequency. Typically, the mains frequency is for instance 50 Hz (Europe) or 60 Hz (USA), and consequently the LED light output may have a ripple frequency of 100 or 120 Hz. This is observable, and therefore it is desirable that the magnitude of the ripple current is as low as possible.
Further, the power drawn from the mains must be proportional to the power consumed by the load L, and this is achieved by adapting the duty cycle of the switching control, wherein an increase in the relative duration of the first state corresponds to an increase in power.
A typical approach in known single-stage PFC LED drivers is to place the buffering, or 100 Hz/120 Hz flicker filtering, at the output of the DC/DC converter because placing significant buffering at the input of the converter would depreciate the power factor and increase the line current distortion. The output buffer typically consists of a large output capacitor C2 which forms a time constant with the dynamic resistance of the LEDs. To improve LED efficacy, LED manufacturers have consistently reduced the dynamic resistance of LEDs over the last decade, which has caused the required output buffer size and cost to increase significantly.
An alternative is to provide feedback control of the converter output current so that the current ripple is prevented in a dynamic way. Two output current regulation methods are commonly used with BiFRED converters used within LED Drivers.
A first is peak current control. This method involves detecting the peak current at the primary side of the converter. Peak current control does not provide a constant output current with a BiFRED converter, but the method can be implemented with low component count and it can obtain middle level line/load regulation. However it is very difficult to meet 4% current ripple although using a large bulk capacitor.
A second method is closed loop control with secondary side current detection. A circuit for this closed loop method is shown in FIG. 2. The same references are used as in FIG. 1 for the same components.
The output current is sensed using a sense resistor R1 and the measured voltage across the sense resistor R1 is compared to a reference Vref at the secondary side of the converter using an error amplifier 20. The output of the control loop is passed to the primary side of the converter through an isolation barrier such as an opto-coupler 22.
Provided that the error amplifier 20 has sufficient open loop gain and very fast response time, this method provides very low output current ripple, so it is for example easy to meet a 4% ripple requirement. Unfortunately providing isolation in the control loop is quite costly.
There is therefore a need for a BiFRED converter which enables control of the output current ripple with a simple and low cost circuit.